1. Field of the Invention
The present invention relates to a horizontal continuous casting process, and more particularly to a method of smoothly and efficiently treating a continuously cast strand (c.c strand) in a final stage in the casting process.
2. Description of the Prior Art
FIG. 1 shows a schematic longitudinal sectional view illustrating a horizontal continuous casting state as an example of the prior art. In the figure, molten steel M within a tundish 1 is drawn intermittently through a tundish nozzle 2, a feed nozzle 3 arranged in coaxial relation with the tundish nozzle 2, a connecting refractory material 4 and a mold 5 to the left in the figure. During the drawing process, the molten steel M is sequentially solidified from the exterior of the steel towards the interior. Such horizontal continuous casting technology has a problem in the treatment of the c.c. strand at a final stage. The final stage includes the stage where the amount of molten steel remaining in the tundish 1 becomes relatively small and the level of the molten steel begins to become lower than the upper surface of the tundish nozzle 2. In this stage, since the pressure effect from molten steel in the tundish is lost, defects in the c.c. strand may occur and in an extreme case breakage of the solidified shell may cause leakage of molten steel. In order to avoid such problem, the following various methods have been proposed. These methods have respective disadvantages as hereinafter described and therefore cannot entirely comply with requirements in practical use.
The first conventional method is a method in which the drawing speed of the c.c. strand is reduced at the final stage of casting and drawing is performed while preventing leakage of molten steel caused by insufficient solidification.
In this method, since the lack of a pressure effect in itself is not eliminated, the defect of a long cavity may be produced in the final c.c. strand and the yield can be reduced. Moreover, the solidification front is transferred to the feed nozzle and therefore the refractory material at the front nozzle may be significantly broken and a solidified substance (fin) may penetrate to the broken portion so as to cause dammage to the mold and in an extreme case may prevent draining to occur.
A second conventional method involves a method in which the tundish nozzle is closed by a shutter mechanism or a stopper in the final stage of casting and any molten steel remaining in the mold is drawn at low speed during solidifying of the molten steel.
Installation of a closing mechanism such as a shutter mechanism or a stopper in the tundish nozzle is quite difficult from a structural viewpoint. Moreover, since the tundish nozzle is broken after each casting process and then reconstructed, installation of such a complicated closing mechanism in each reconstruction is quite uneconomical and not practicable.
A third conventional method concerns a method in which the drawing of the c.c. strand is stopped at the final stage of casting, molten steel in the mold and the tundish nozzle is completely solidified the c.c. strand is then cut at the outlet of the mold, and the c.c. strand remaining in the mold is removed from the mold after being completely cooled.
In this method, the c.c. strand and base metal in the tundish are connected and therefore subsequent treatment is troublesome. Moreover, if the c.c. strand is overcooled, the density of the c.c. strand is increased and bending deformation is apt to occur thereby damaging the inner surface of the mold when the remaining c.c. strand is taken out of the mold.
The above-mentioned disadvantages in the treatment of the final c.c. strand occur frequently when stainless steel with a c.c. strand of a high density is cast in a horizontal continuous casting process, and more or less also occurs in the case of general steel material.